1. Field of the Invention
The invention relates to a hardening accelerant for Portland cement which, in particular, is active even at low temperatures.
2. Prior Art Statement
Calcium chloride is the most effective and longest known settling accelerant which acts even at low temperatures. However, its presence must be limited to 1 to at most 2% by weight of the cement content, since otherwise corrosion of the reinforcing rods is likely and construction damage results.
U.S. Pat. No. 2,390,138 proposes sodium nitrate for use as a settling accelerant. Commercially available additives contain calcium nitrate, frequently combined with triethanolamine (Betonwerk+Fertigteiltechnik, issue 8/1971, page 577).
German Offenlegungschrift No. 2,703,182 recommends the combination of an alkali metal nitrate with a water-soluble compound in the form of a condensation product of sulfonated aromatic hydrocarbon and aliphatic aldehyde and/or alkali metal salt of the condensation product, the alkali metal nitrate content being higher than the condensation product content.
U.S. Pat. No. 4,337,094, finally, proposes a hardening accelerant which essentially comprises calcium nitrate, a polyalkanolamine, for example triethanolamine, and if desired an alkali metal salt or an alkaline earth metal salt of an aliphatic polyhydroxy compound, for example sodium glucoheptonate, and the sodium salt of ligninsulfonic acid.
Further solidification accelerants are based on condensation products of naphthalenesulfonic acid and formaldehyde (U.S. Pat. Nos. 1,972,207, 2,141,569 and 2,478,831) to which still other additives are added (U.S. Pat. Nos. 3,359,225, 3,537,869).
The use of ammonium salts, for example ammonium nitrate, as hardening accelerants is subject to reservations, since ammonia evolves at the high basicities of cement slurries.
All the agents hitherto proposed either have harmful side effects, in particular on the reinforcing steels, or, in particular at low temperatures, must be used in excessively high concentrations for full activity. It is therefore an object to provide a hardening accelerant for Portland cement mortar which does not have the disadvantages described.